Commercial nuclear reactors used for generating electric power include a core composed of a plurality of fuel assemblies which generate heat for electric power generation purposes. Each fuel assembly includes an array of fuel rods which are held in a spaced relationship with each other by means of spacer grids of egg-crate configuration spaced along the fuel assembly length. The fuel rods may be approximately 0.5 inch in diameter and about 12 feet long, thus requiring a number of supporting grids along their length.
As discussed in U.S. Pat. No. 5,024,807 to Hatfield et al., the disclosure of which is herein incorporated by reference, metallic debris in the coolant which collects or is trapped in fuel rod spacer grids adjacent to the fuel rod cladding is believed to be responsible for a significant percentage of known fuel rod failures. Traditional fuel assembly designs were known to sustain a distribution of debris-induced failures that clearly showed the lowest spacer grid to be a very effective filter for debris. Unfortunately, the short, lower end caps on the fuel rods of such fuel assemblies ensured that the hollow cladding tubes would be adjacent to the trapped debris, and that any flow-induced motion of the debris could wear through the thin wall of the tubes and cause rod failure. In traditional fuel assemblies, the lowest spacer grid was some distance up from the bottom of the fuel rod, since, in the absence of a positive axial capture device for the rod, the grid needed to be located at an elevation where it always laterally captured a "lifted" rod. Rods could potentially lift in response to coolant flow during abnormal conditions. One choice for a debris-resistant fuel assembly design is to merely lengthen the solid end cap such that it is extends up through the bottom spacer grid. This simplistic solution, however, is not feasible because zirconium alloy bar stock used for end caps is very expensive and because void volume within the fuel rod and/or the active fuel length would be negatively affected.
In response to this observation, Hatfield et al. designed a fuel assembly with a spring detent spacer grid of intersecting strips. The spring detent spacer grid allowed the grid to be moved downward, thereby reducing the solid zirconium alloy material length required. To preclude the "rod lift", the grid included a fuel rod capturing spring detent device. This device engaged a circumferential groove with tapered sides in the fuel rod end cap which created enough axial restraint to prevent or minimize rod lift under all flow conditions, but not enough restraint to significantly affect fuel rod reconstitution. In addition, integral leaves substantially symmetrically arranged on either side of the strip intersections were also added to greatly increase the likelihood that debris that passed the novel first or bottom spring detent spacer grid was too small to become trapped at a higher grid where it could damage the cladding of the active fuel region.
Despite these advantages, the spring detent feature was discovered to restrict rod replacement somewhat as compared to a standard lower spacer grid without a detent feature by requiring more pulling force to be applied to the rod during reconstitution.
Other spacer grids make use of conventional springs to hold fuel rods in place. For example, as discussed in U.S. Pat. No. 4,389,369 to Bryan, the disclosure of which is herein incorporated by reference, grid designs commonly include interwoven Inconel or Zircaloy straps which form multiple cells, each cell having springs on two adjacent walls and projections or dimples on each of the other two walls. The springs laterally impress resistive forces on each fuel rod in the assembly. Although this fuel assembly design performs exceptionally well in a nuclear reactor, one disadvantage inherent in the design is that the inwardly projecting springs and dimples occasionally mar or score the surface of fuel rods while they are being pulled into the fuel assembly grids. In carrying out this fuel rod loading operation, the grids are immovably held in position while a longitudinal steel rod attached to the end of a fuel rod pulls the fuel rod axially through the aligned openings, or cells, in the grids. As the rod engages the springs and dimples in the grid cells, their edges engage the exposed, relatively soft surface of the moving fuel rod and, in some cases, score its surface sufficiently deep as to cause the rod to fall outside established fuel rod surface specifications.